In the field of cable communication transmission systems, it is often necessary to transmit over a coaxial cable multiple signals, separated from one another in different frequency bands. In many such systems, it is desirable to transmit signals simultaneously in both directions along the cable, the signals travelling in opposite directions being separated from each other through the use of two separate frequency bands. An example of such a system is found in the CATV field, wherein signals carrying a great number of complete television and FM programs may be transmitted either way on a single cable. For example, in one common type of system, one band of frequencies ranging generally from 5 MHz to 35 MHz and called the sub-frequency may be travelling in one direction on the cable, while another band of frequences ranging generally from 50 to 300 MHz, called the VHF band, may be travelling in the other direction.
Because of attenuation losses in the cable, it is necessary to provide amplifiers at intervals along the transmission line. It is known in the art to provide diplexing filters at each amplification station along the line, so that each band of frequencies may be separated from the composite signal on the line, amplified, then re-introduced to the line with another diplexing filter.
Prior art diplexing filters comprise separate high and low pass filters both coupled to the transmission line, with each filter providing an approximate impedance match to the line over its own range of frequencies. One problem which exists with these prior art diplexing filters is that they do not provide an impedance match for the transmission line for frequencies near the crossover between the high and low pass filters. Thus, any power having a frequency near the crossover frequency of the system which is inadvertently introduced into the transmission line is not absorbed, but is reflected back and forth along the line, thereby creating undesired effects. Even though the communication transmission system is carefully designed so that the upper and lower frequency bands are spaced from the crossover point, in practice undesired frequencies near the crossover are still introduced due to harmonics and sub-harmonics generated by nonlinear devices present in the system, and by noise and electromagnetic interference picked up by the system.
There exists in the prior art one class of diplexing filter which theoretically could solve the above-mentioned problems by providing a constant input impedance at all frequencies. This class of filters is generally called hybrid diplexing filters. Unfortunately, despite their theoretic advantages, hybrid diplexing filters have not achieved practical commercial usage, to the best of our knowledge, because certain problems involved in making a practical physical realization of the theoretic circuit have not heretofore been successfully overcome.
Hybrid diplexing filters generally comprise more or less symmetrical bridge circuits made up of a pair of high pass filters and a pair of low pass filters, with a common port and low and high pass ports connected at appropriate nodes in the bridge. The common port is generally connected to the transmission line, so that signals inthe higher frequency band are coupled from the transmission line to the high pass port, in either direction. Similarly, signals in the lower frequency band are coupled from the transmission in the to the low pass port in either direction. Signals can therefore be split off from the composite signal on the line, for purposes of amplification, and after amplification they can be re-introduced to the line using another such filter.
In designing hybrid diplexing filters, the value of the components in the high and low pass filters are calculated to provide a crossover point between the two bands of frequencies of interest in a given system. A main purpose of these diplexing filters is to maintain a constant input impedance at the common port while providing a high degree of isolation between the low pass and high pass ports over the entire range of frequencies to be transmitted. In order to obtain useful efficiency, this input impedance must match very closely the driving point characteristic impedance, e.g. the transmission line being used, or else reflections will occur between the transmission line and the diplexing filter, leading to excessive degradation and signal losses in the system. A measure of the efficiency of energy transfer between the cable and the filter is the return loss, which is directly dependent upon the input impedance of the filter. Ideally, this return loss should be as low as possible, preferably less than -20 db to -30 db.
The mathematical theories of design of hybrid diplex filters are known in the art as set forth, for example, in U.S. Pat. No. 3,593,209, issued to N. C. Gittinger. However, practical devices in the prior art constructed according to these principles have failed to provide good performance; especially at the higher frequencies. Specifically, prior art devices have exhibited excessively large return loss which makes them unusable or only marginally usable in a practical CATV system. These prior art hybrid diplexing filters exhibit a fall-off of impedance at the higher frequencies which accounts for the poor return loss. For example, in systems designed for 75 ohm cables, typical prior art diplex filters may have a return loss of better than -20 db below 100 MHz, but this figure will fall to approximately -20 db by 150 MHz, and may be down to only -14 db at 300 MHz. This characteristic not only results in a significant loss of signal due to impedance mismatching, but also can lead to severe "ghosting" of the video signals being transmitted. These last-mentioned effects are particularly detrimental to the signals carrying color information in a television signal.
The main reason for this poor performance of prior art hybrid diplexing filters is stray capacitance which is associated with any physical realization of a circuit. Sources of stray capacitance include the coaxial connectors, adjacent conductor paths on a circuit board, and capacitive effects between windings in a coil, or between adjacent coils. Standard techniques of physical circuit layout to minimize capacitances have proved to be ineffective in dealing with this problem, in part, because of the high frequencies involved.
One prior art technique for attempting to compensate for some of this stray capacitance involves placing an additional inductance, called a peaking coil, in series between the common port and the bridge circuit and/or in series with the first high pass filter section. This technique helps to compensate for the capacitance by boosting the input impedance over a range of high frequencies. While this method has met with limited success, it suffers from the drawback of requiring additional components, thereby incurring additional assembly costs, and also may involve production problems in maintaining uniformity of performance between production units. This is because the peaking coils are of very low inductance, perhaps only one-quarter of a turn of wire, so that slight non-uniformities in assembly may have a pronounced percentage change on the effect of the peaking coil.
It is also desirable to provide power for operating the amplifiers along the line through the transmission line itself. This would involve placing 60 cycle AC power on the transmission line in addition to the upper and lower signal bands. The object is then to provide means at each amplification station for taking the 60 cycle power from the transmission line, rectifying it and using the resulting DC potential for powering the amplifiers. One problem which must be overcome before such a scheme can be useful in a practical system is the problem of isolation of the signal circuits, particularly the lower signal frequency circuits from the power circuits, and vice-versa. One alternative is to connect an rf stop band filter circuit to the common port. However, because the rf filter must pass a rather large amount of power, this leads to a coil for the rf filter which has a significant amount of interwinding capacitance, and this capacitance can in itself load down the common port, degrading the impedance match at high frequencies.
The present invention provides means for compensating for unwanted stray caapacitances associated with a physically realizable circuit, and also provides means for receiving low frequency power from the transmission line, without upsetting the high frequency signal handling properties of the filter.
According to one aspect of the present invention, the necessary impedance compensation for stray capacitances is accomplished by including impedance compensation means within the high pass filter which connects the common port to the high pass port. The physical layout of the circuit is carefully controlled so that the stray capacitances associated therewith are held to a reasonably small value. The impedance compensation means is then selected to provide the necessary compensation to compensate for the remaining stray capacitance. In a preferred embodiment, this impedance compensation is provided by including a transformer in this high pass filter. The transformer performs a dual function. One function is known in the prior art, and this function is to provide phase inversion so as to provide power isolation between the low and high pass ports which are opposite each other across the bridge. This is necessary to prevent unwanted oscillations which might occur in a system including amplifiers if the loop gain through the amplifiers and diplexing filters became greater than one at a given frequency. Although this particular function of the transformer is known in the prior art, prior art circuits placed the phase inverting transformer in a leg away from the common and high pass ports.
The second function of the transformer according to a preferred embodiment of the invention is to provide a carefully predetermined amount of leakage inductance in this high pass filter. This leakage inductance will then compensate for the effects of the unwanted stray capacitance. This use of the transformer is not found in prior art hybrid diplexing filters.
According to another aspect of the present invention, one way to control the leakage inductance of the transformer is to use a modified bifilar winding technique. The primary and secondary conductors are first twisted together, then wound on a core to form the transformer. The degree of twisting determines the amount of leakage inductance.
According to yet another aspect of the present invention, means are provided for taking low frequency power from the hybrid filter for line powering the amplifiers without upsetting the carefully balanced high frequency performance of the circuit.
According to a preferred embodiment, this is accomplished by passing the low frequency power through at least a part of the low pass filter which communicates between the common port and the low pass port. The coils in at least a part of this low pass filter are wound with sufficiently thick wire so that the low pass filter can pass both the lower band of rf signals, and the low frequency power. Power blocking means associated with the low pass port provides isolation of the low pass port from the low frequency power, and rf signal isolation means associated with the power passing port isolates the power passing ports from rf signals.